Galaxies 09: Assembly, Gas Content and Star Formation History of Galaxies

Andreea Petric
Spitzer Science Center /Caltech

MIR Diagnostics and Warm Molecular Gas in Local Luminous Infrared Galaxies in the GOALS Sample

We present spectra taken with the Infrared Spectrograph on Spitzer covering 5-38 $\mu$m region of 248 nuclei of luminous infrared galaxies (LIRGs) found as part of the Great Observatories All-sky LIRG Survey (GOALS) to isolate the AGN versus starburst contribution to the MIR emission and the fate of molecular warm and cold gas along a merger sequence of LIRGs. The high ionization lines [NeV] and [OIV] suggest that only 2\% of LIRGs are AGN dominated. The EQWs of the 6.2 PAH feature indicate that 16\% of sources are AGN dominated. Diagnostics based on the shape of the MIR continuum imply that 19\% of the sources have a hot dust component which may be associated with an AGN. No obvious correlations are found between the AGN contribution to the IR and the merger stage or luminosity of these galaxies in contrast to what is found in ULIRGs. The 6.2 PAH EQW are used to estimate an AGN contribution of 14\% to the IR power in those sources. We use observations of the pure rotational transitions of molecular hydrogen S(0) (28.22 $\mu$m), S(1) (17.04 $\mu$m) , S(2) (12.28 $\mu$m) and S(3) (9.67 $\mu$m) to determine the mass and temperature of the warm gas. We detect the H$_{2}$ S(0), S(2) and S(3) transitions in about 30-40\% or all sources and S1 in 94\% of our objects. We find a wide range of ratios of the H$_{2}$ to the IR luminosities between $\sim 7 \times 10^{-5}$ and $1.7 \times 10^{-3}$ with a median of $2 \times 10^{-4}$. We find a weak correlation between the warm H$_{2}$ mass and IR luminosity. The derived H$_{2}$ temperatures seem to change with merger stage in that more advance mergers appear to have higher temperatures. We compare the results obtained for the GOALS galaxies to similar studies of nearby galaxies and ULIRGs.

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